@frostion
I was using engine 1.9.0.0.9687. I assume that wasn't new enough for Iron war. If I update the engine., older games that are not finished will not run. Is that right? At least, that's been my experience.
I really enjoy the game and I appreciate all the effort that went into it.

@forthebirds There was a rare bug where a battle that completed might not be cleared from the battle queue in the engine causing it to become stuck when trying to start another battle: https://github.com/triplea-game/triplea/issues/3377. This has been fixed in the pre-release so shouldn't happen in newer engine versions. Unfortunately, upgrading you engine version to a pre-release version won't fix an existing save game.

I'd recommend opening your save game then the history panel. Then right click to take a save game before the Southern Congo battle was ever fought. You should then be able to replay the turns and shouldn't hit the issue (it is very rare and essentially a race condition so shouldn't ideally happen again). If that doesn't resolve it then let me know and I'll take a closer look.

@redrum
I'm sorry to say that the problem doesn't seem to be resolved.
I saved the game at the beginning of South Africa's turn & started from there. South Africa's turn was computer operated.
I then took my turn again & the same error message popped up that I needed to finish a battle first which I didn't even try to do this time. Just thought that you would like to know.